1. Field of the Invention
The present invention relates to automatic transmissions for controlling the transmission based on the vehicle speed and the throttle opening.
2. Description of the Prior Art
FIG. 4 shows a conventional automatic transmission. A well known parallel axis gear type transmission 1 is controlled by a transmission actuator 2 which consists of a select actuator 2a and a shift actuator 2b. Both of the actuators 2a and 2b are driven by a hydraulic drive system 3 which consists of a tank 3a, a pump 3b, an accumulator 3c, and a hydraulic pressure switching electromagnetic valve (not shown) to control the drive gear of the transmission 1 via a pair of piston rods 2c and 2d. The actuator 2 is controlled by a drive unit (D/U) 2e, which in turn is controlled by a main control unit 4. Feedback control is made such that the actual position signals of a pair of potentiometers 2f and 2g agree with the target position signals M fed from the main control unit 4. Consequently, as shown in FIG. 5, the gear position is shifted from the neutral position 5g to the first gear 5a, second gear 5b, third gear 5c, fourth gear 5d, fifth gear 5e, or reverse driving 5f. A clutch 6 is linked to the piston rod 7a of a clutch actuator 7 for engagement and disengagement. The hydraulic drive system 3 drives the clutch actuator 7. The actual position of the piston rod 7a is detected by the potentiometer 7c, and the actual position signal is fed back to a drive unit 7b. The drive unit 7b is controlled by the main control unit 4 such that the actual position signal agrees with the target position signal F fed by the main control unit 4.
An engine 8 has a control unit 9 which is controlled by the main control unit 4. The rotation of the engine 8 is transmitted to an axle 10 via the clutch 6 and the transmission 1. Respective signals of an accelerator opening detector 11, a brake pedal detector 12, a key switch 13, an exhaust brake switch 14, and a selector 15 and the rpm output signal W of a vehicle speed detector 22 are fed to the main control unit 4 to control the drive units 2e and 7b and the engine control unit 9, a display panel 16, and a control unit 17a for controlling a gear position display panel 17. The rpm output signal W from the vehicle speed detector 22 is multiplied in the main control unit 4 by a constant which consists of the radius r of car wheels 23 and the gear ratio of a differential gear 24 to provide a vehicle speed V, which is displayed on an instrument panel (not shown) along with the engine rpm and the fuel reserve. Thus, the main control unit 4 controls the clutch 6 and the transmission 1 based on the throttle opening S or accelerator pedal depression, the vehicle speed V, and the position of the selector 15. The gear position is shifted from the neutral 5g to one of the shift positions 5a-5f as shown in FIG. 5.
As Japanese Patent Application Kokai No. 59-1855 discloses, the main control unit 4 includes an automatic shift unit 30 for effecting automatic shifting according to input signals such as the vehicle speed and the throttle opening. The automatic shift unit 30 includes the first and second gear shift maps 30a and 30b having a gear shift characteristic N which is determined by the vehicle speed V (rpm W.times.constant) and the throttle opening S (accelerator pedal depression) as shown in FIG. 6, a computer unit 30 c for computing the gear position of the transmission 1 based on the gear shift characteristic N of the first or second gear shift maps 30a or 30b, and a gear selector 30d for controlling the actuator 2 based on the gear position from the computer unit 30c to make upshift or downshift. When the selector 15 is set in the automatic shift position "4", the automatic shift unit 30 controls the transmission 1 between the first and fourth gears based on the first gear shift map 30a while when the selector 15 is set in the automatic shift position "5" it controls the transmission 1 between the second and fifth gears based on the second gear shift map 30b.
For example, when the accelerator pedal is released to reduce the vehicle speed, with the transmission 1 in the second gear (point A of the gear shift map 30a), the automatic shift unit 30 upshifts the transmission 1 to the third gear despite the fact that the vehicle speed is reduced, thus failing to provide satisfactory engine braking.
Japanese Patent Application Kokai No. 58-211061 discloses an automatic transmission which is able to detect the rate of accelerator pedal depression and, if the rate is above a certain value, prohibit any shift. Thus, if the rate of accelerator pedal change is above the certain value, no upshift from A to B is made so that the transmission 1 remains in the second gear, thereby providing satisfactory engine braking.
In the conventional automatic transmission, when the driver tries to apply engine braking by releasing the accelerator pedal at the certain rate, the transmission can be upshifted because the perception of the certain rate varies with the individual driver. In addition, to detect the certain rate, it is necessary to provide a differential circuit for differentiating changes of the accelerator pedal. This differential circuit is so sensitive to noise that it suffers from frequent malfunctions.
When the accelerator pedal is released abruptly after the vehicle starts to move with low speed gear and half clutch, the clutch is kept disengaged to prevent engine stop when the vehicle stops while the automatic transmission is put into the upshift prohibit condition. As a result, when the vehicle starts to descend downhill, for example, no upshift is made while the clutch is kept disengaged even when the vehicle speed is increased, thus presenting very dangerous running conditions. When the accelerator pedal is depressed under the upshift prohibit condition, the upshift prohibit condition is released instantly, failing to provide satisfactory acceleration despite the fact that the accelerator pedal is depressed for acceleration.